1. Field of the Invention
The present invention relates to a pneumatic tire for an airplane which is used on an airplane such as a jet passenger airplane or the like.
2. Description of the Related Art
High-speed traveling on runways is required of pneumatic tires for airplanes. Thus, in order to further strengthen the hoop effect of a belt layer, a reinforcing layer which includes a plurality of cords is provided at the outer side of the belt layer in the tire radial direction.
In order to strengthen the hoop effect of the belt layer even more, it is preferable to form the belt layer such that the angle formed by the cords of the reinforcing layer with respect to the tire circumferential direction is substantially 15° or less.
Further, in order to eliminate the cut-ends of the cords of the reinforcing layer, it is preferable to use a reinforcing layer in which the cord direction is substantially the circumferential direction (a so-called spiral structure or jointless structure).
In a reinforcing layer including cords which are set at an angle of 15° or less with respect to the tire circumferential direction, there is usually one or fewer joint portion at which the cords are cut. Namely, the cords of the reinforcing layer are connected over substantially one circumference in the tire circumferential direction.
There have been cases in which, while an airplane is travelling on a runway, the airplane rides over a foreign object such as a metal piece or the like, such that many of the plies of the tire are cut and the tire on occasion bursts.
In a case in which the tire bursts while the airplane is taxiing at low speed, taking-off can be stopped and the tire can be replaced. However, when the airplane cannot stop during taking-off or when pilot does not notice the burst, the airplane takes off and lands on a tire at which a large number of plies are cut and damaged or on a tire which has burst.
There have been cases in which, when an airplane travels at high speed on a tire at which a large number of plies have been cut and damaged or on a tire which has burst in this way, the tire breaks into pieces due to centrifugal force and pieces of the tread scatter about.
In the process in which the tread pieces are formed, a considerable force is needed in order to cut the cords of the reinforcing layer. Thus, it is usually the case that cracks advance along the cords of the reinforcing layer.
Accordingly, in a tire in which the angle formed between the cords of the reinforcing layer and the circumferential direction of the tire is substantially 15° or less and thus the cords are disposed so as to be long along the circumferential direction, the tread pieces which scatter are also long. Here, a problem arises in that such long tread pieces which scatter could cause great damage to the airplane, with the centrifugal force caused by the high speed traveling, although these tread pieces are simply a mass of rubber and cords.
On the other hand, when the angle formed by the cords of the reinforcing layer with respect to the tire circumferential direction is greater than 15°, in a usual pneumatic tire for airplanes, the length of the cords of the reinforcing layer in the circumferential direction is about ⅓ of the circumference of the tire. Accordingly, at the time of separation, if the separation occurs across the entire tread surface without any cutting of the cords of the reinforcing layer, the size of the tread pieces is limited to about ⅓ of the circumference of the tire. However, the hoop effect of the belt layer by such a reinforcing layer (a reinforcing layer in which the angle formed by the cords of the reinforcing layer with respect to the tire circumferential direction is greater than 15°) is poor, which is not preferable.